Semiconductor lasers are formed of multiple layers of semiconductor materials. The conventional semiconductor diode laser typically includes an n-type layer, a p-type layer and an undoped layered active structure between them such that when the diode is forward biased electrons and holes recombine within the active structure with the resulting emission of light. The layers adjacent to the active structure typically have a lower index of refraction than the active structure and form cladding layers that confine the emitted light to the active structure and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge-emitting or surface-emitting.
A semiconductor laser that emits photons as electrons from within a given energy band cascade down from one energy level to another, rather than emitting photons from the recombination of electrons and holes, has been reported. See, J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science, Vol. 264, pp. 553, et seq., 1994. This device, referred to as a quantum cascade laser (QCL), is the first reported implementation of an intersubband semiconductor laser. The basic light-generation mechanism for this device involves the use of 25 active regions composed of 3 quantum wells each. Injection by resonant tunneling occurs in the energy level (level 3) of the first, narrow quantum well. A radiative transition occurs from level 3, in the first well, to level 2, the upper state of the doublet made by two coupled quantum wells. Quick phonon-assisted relaxation from level 2 to 1 insures that level 2 is depleted so that population inversion between levels 3 and 2 can be maintained. Electrons from level 1 then tunnel through the passive region between active regions, which is designed such that, under bias, it allows such tunneling to act as injection into the next active region. Further developments of this type of device are reported in F. Capasso, J. Faist, D. L. Sivco, C. Sirtori, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, Conf. Dig. 14th IEEE International Semiconductor Laser Conference, pp. 71-72, Maui, Hi. (Sep. 19-23, 1994); J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett., 66, 538, (1995); J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, “Continuous wave quantum cascade lasers in the 4-10 μm wavelength region,” SPIE, Vol. 2682, San Jose, pp. 198-204, 1996; and J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Room temperature mid-infrared quantum cascade lasers,” Electron. Left., Vol. 32, pp. 560-561, 1996. More recently continuous wave (CW) operation has been achieved at 300 K, but with very low power conversion efficiency (<0.5%) and only at wavelengths around 9 μm. See M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, Science, Vol. 295, pp. 301-305, 2002.
Despite this rapid improvement in the performance capabilities of GaInAs/InP-based quantum cascade lasers, it is unlikely that they will ever be able to operate CW at 300 K with high power conversion efficiency (>15%) at wavelengths of interest in the mid-infrared (3 to 5 μm) and far-infrared (8 to 12 μm) wavelength ranges due primarily to the fact that their radiative efficiency is inherently poor. This poor efficiency is quantified by noting that the non-radiative LO-phonon-assisted relaxation time for the upper laser states is about 1.8 ps and the radiative relaxation time is 4.2 ns. One approach to obtaining room temperature CW operation of intersubband semiconductor lasers in the mid-infrared (3 to 5 μm) and far-infrared (8 to 12 μm) ranges involves the use of two-dimensional arrays of quantum boxes, with each quantum box incorporating a single-stage, intersubband transition structure. See C-F Hsu, J-S. O, P. Zory and D. Botez, “Intersubband Quantum-Box Semiconductor Lasers,” IEEE J. Select. Topics Quantum Electron., Vol. 6, 2000, pp. 491-503; U.S. Pat. No. 5,953,356 entitled “Intersubband Quantum Box Semiconductor Laser.”
Room temperature intersubband emission has been reported for single-stage, unipolar devices only from InP-based structures at wavelengths as short as 7.7 μm. C. Gmachl, et al., “Non-Cascaded Intersubband Injection Lasers at λ=7.7 μm,” Appl. Phys. Left., Vol. 73, 1998, pp. 3822-3830. For 30- to 40-stages, GaAs—AlGaAs quantum cascade lasers at room temperature, intersubband emission wavelengths shorter than 8 μm cannot be achieved, since at higher transmission energies, the active-region upper level is apparently depopulated via resonant tunneling between the X valleys of the surrounding AlGaAs barriers. C. Sirtori, et al., “GaAs—AlGaAs Quantum Cascade Lasers: Physics, Technology and Prospects,” IEEE J. Quantum Electron., Vol. 38, 2002, pp. 547-558. Optimization studies of GaAs-based devices have shown that for thin barriers between the injector region and the active region, two effects occur which cause significant decreases in the upper level injection efficiency: (1) a diagonal radiative transition from injector-region ground level, g, to an active region lower level, and (2) severe carrier leakage from the level g to the continuum. S. Barbieri, et al., “Design Strategies for GaAs-based unipolar lasers: optimum injector-active region coupling via resonant tunneling,” Appl. Phys. Lett., Vol. 78, 2001, pp. 282-284. In addition to these limitations, quantum cascade lasers are conventionally formed of three regions, a superlattice injector, an active region, and a superlattice reflector/transmitter, functioning as a distributed Bragg reflector (DBR), which is identical in structure to the superlattice injector. Fabrication of devices with complicated structures of this type is both difficult and relatively expensive. Furthermore, for such devices the necessary impurity doping in the superlattice injectors causes a significant increase in the room-temperature threshold current density due to excited carriers from the superlattice reflector/transmitter regions that fill the lower levels of prior active regions, thus reducing the population inversion.